General Description of the Manufacturing Process of Laminated Hardwood Floors
Currently, high-strength hardwood floors are manufactured by cutting hardwood planks into small strips and subsequently reassembling the strips into 12 in. wide 50 ft. truck floor planks. Before being ripped into small strips, the hardwood plank needs to be sanded or planed on both its top and bottom surfaces. Usually, a rough 4/4 dried hardwood plank has a thickness of between 1 inch to 1.125 inch. During the sanding or planing process, the thickness of the board is reduced and will reach generally between 0.80 and 0.96 inches. It should be noted however that the surface preparation can be done before the hardwood plank is ripped or after. This process is necessary because it prepares the surface of the wood where the glue will be applied. Indeed, the width of the small strip will become the thickness of the laminated floor. The top and the bottom of the plank are the parts where glue will be applied. The surfaces need to be smooth and even on both sides with little tolerance (5 or 7 mils) between both surfaces. The preparation of the wood surfaces is very important. Improperly prepared wood surfaces will create problems during the gluing process or undesirable defects on the final product.
The next phase of manufacture is to remove the defects in the sticks. Some defects such as knots, crack, flash, bark, etc., are the result of variations in the raw wood supply and are removed for both structural and cosmetic reasons. Other defects can be the result of lack of processing during the preparation of the wood surfaces. During the wood surface preparation, for some reason, some surface areas of the sticks have not been hit properly by the sandpaper of the sanding machine or knives of the planer. Those areas are called “hit or miss defects”, which means that the sticks do not have the adequate surface preparation to receive the glue. Those areas have to be removed as a defect or the sticks themselves having that defect are removed and must be re-sanded or re-planed in smaller dimensions before being used further in the process into a special batch using narrower sticks.
Once the strips are cleaned of their defects, glue is applied on the appropriate surfaces and then they are glued together using an appropriate adhesive that is cured in a high frequency press. After the assembled plank exits the high frequency press, it is planed to final thickness. At this phase in the manufacturing operation, cosmetic defects are repaired and the boards are prepped for painting.
The prepared boards are then sent through an automated painting line where they are coated with approximately 3 to 4 mils of water-based paint.
Method to Remove Defects on the Sticks
Presently, in North American manufactured laminated floors, the defects in the wood sticks are removed manually by a highly skilled and trained person. With a cross cutter saw operated manually, the skilled person cuts the wood on each side of the defects to remove the defects from the sticks.
But there is a possibility that the defects can be also removed automatically by using new technology which uses a scanner and an automated cross cutting saw. The scanner detects (with sensors such as a camera, X-ray, micro wave, ultrasound, infrared laser, etc.) the defects on the sticks. The computer to which the scanner is operatively connected sends the information (location of the defect on the sticks) to an automated cross cutting saw which is adapted to remove the defects previously detected by the scanner.
The scanner can use different methods to detect hit or miss defects. One of the methods is to detect hit or miss defects by measuring the dimensions of the sticks. This measurement can be done using optical triangulation technology using laser sensors. If the dimension (in that the case the thickness of the stick) is more or less than a predetermined target, the scanner will deduce that there is a hit or miss defect. Another method consists in detecting the hit-or-miss defect with a camera. In this method, the color tone variation between the hit-or-miss defect and the immediate area next to it provides an indication of the defect. This technique will deduce a hit-or-miss defect on the stick when there is a variation of color tone on the wood surface. Finally, the hit-or-miss defect can be also detected by applying, manually, coloured marks on the sticks before the sticks go into the scanner. The sensor (usually a camera) in the scanner will recognize the mark (not the defect itself) and it will be identified as an area where there is a defect to be removed. All these techniques can be used separately or in combination.
These techniques have some problems which limit the efficiency of the detection of the hit or miss defect on the sticks.
When the dimension approach is used, problems occur when the hit-or-miss defects are in the range of the tolerance of the sanding or planning process.
Assuming that the tolerance of the sanding process is around 7 mils, at the exit of the sanding machine the thickness of the sticks will be between 0.916 and 0.93 inches. If the scanner is adjusted to not recognize as a hit or miss defect any variation under 7 mils and that some hit or miss defects are in that range, missed detection will occur (some hit or miss defects will not be detected and will not be removed). On the other hand, if the scanner sensitivity detection is increased above 7 mils, the false detection will occur creating an increase of waste (the scanner will detect hit or miss defects incorrectly).
In the case that the color tone variation method is used to detect hit-or-miss defect, problems occur when color tone variation related to hit-or-miss conflicts with the natural coloration of the wood itself. It is extremely difficult for a camera (black and white) to distinguish between variations of tone of the natural coloration of hardwood and variations of tone due to a hit or miss defect.
In the case when coloured marks are applied manually, the extra labour cost decreases the advantages of using the scanner and cross cutter saw technology. The investment in the automated detection is justified economically by the reduction of the number of people on the production line. If, after making the investment, it takes people to put the coloured marks, the reduction of the labour cost almost disappears. Another problem with the coloured mark is human mistakes which will increase the waste.
For these reasons, the detection of the hit-or-miss defect using these techniques, in the discussed case here, are not accurate, economically inefficient and could create problems: defects which are not removed reduce the quality of the finished product or good wood is identified as defect when it should not be, increasing the waste of the raw material.
Tests have shown that 5% of the sticks at the exit of the automated saw have to be rejected because of the presence of hit or miss defects. At the same time, the improper cuts increase the waste by at least 2%. The false detection of the hit or miss defect can create even more doubt on the economic viability of the use of the scanner in the manufacturing process of hardwood laminated floors or any other manufacturing process in the wood industry where hit or miss defects are important to detect.